Suppression of measurement-induced state transitions in cosφ-coupling transmon readout: Part II. Theory

Previous published work, Dassonneville et al. [1], demonstrates qubit readout with a pure non-perturbative cross-Kerr coupling, the cosφ-coupling, engineered by a transmon molecule circuit. In this work we present a theoretical analysis of a new implementation of that circuit [2]. We demonstrate, using numerical diagonalization and a branch analysis of the dressed spectrum, the robustness of the quantum non-demolition property of the measurement as the photon number in the readout mode is increased. To further characterize this, by tuning in and out of the regime of pure cross-Kerr coupling, we explore the possible measurement-induced state transitions (MIST), in agreement with experimental data. We show that parity symmetry makes the cosφ-coupling more robust to measurement photons than the usual dispersive coupling, making it a compelling alternative for high-fidelity and non-demolition transmon readout.

 

[1] R. Dassonneville, T. Ramos, V. Milchakov, L. Planat, et al., Phys. Rev. X 10, 011045

[2] Cyril Mori et al., arXiv:2509.05126